1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, etc., and more particularly to an image forming apparatus in which an intermediate transfer member is used.
2. Discussion of the Background
A background image forming apparatus includes the following elements: an intermediate transfer member for bearing a toner image transferred from an image bearing member, a primary transfer charger for applying a charge to the intermediate transfer member for transferring the toner image from the image bearing member to the intermediate transfer member, a secondary transfer charger for applying a charge to a final transfer member, e.g. a paper sheet, for transferring the toner image from the intermediate transfer member to the final transfer member and an intermediate transfer member discharging device for removing a charge from a surface of the intermediate transfer member which has transferred the toner image to the final transfer member. In such an image forming apparatus, a surface potential of the intermediate transfer member is made uniform by removing the charge from the surface of the intermediate transfer member with the intermediate transfer member discharging device to prepare for a next primary transferring operation from the image bearing member to the intermediate transfer member.
However, in some cases in the above-mentioned background image forming apparatus, the surface potential of the intermediate transfer member will not be at a desired predetermined potential, or an irregular potential remains after removing the charge from the intermediate transfer member with the intermediate transfer member discharging device. These states of defective discharging deteriorate any subsequent transfer process from the image bearing member to the intermediate transfer member.
As a result of an investigation of a cause of the defective discharging on the intermediate transfer member made by the inventors of the present invention, it has been determined that a main cause of the defective discharging is significant unevenness of the surface potential of the intermediate transfer member whose charge has not been effectively removed by the intermediate transfer member discharging device.
This unevenness may particularly occur if the image forming apparatus is a color image forming device constructed so that the primary transferring operation is performed a plurality of times to superimpose toner images of various colors, which have been formed on the image bearing member, onto the intermediate transfer member, and in which then the toner images superimposed on the intermediate transfer member are transferred to the final transfer member at one time. In this case, the surface potential of the intermediate transfer member whose charge has not been removed by the intermediate transfer member discharging device may be uneven. Further, the degree of unevenness may depend on the number of the toner images superimposed on the intermediate transfer member, in other words, the number of times the primary transfer to the intermediate transfer member is executed.
Further, in such a background image forming apparatus, a transfer condition for transferring the toner image on the intermediate transfer member to the final transfer member is set to be almost at a center of a preferable transfer range in which a transfer rate to the final transfer member is normally set to a predetermined value or greater. A difference between a set value of this transfer condition and an upper limit of the preferable transfer range or a lower limit thereof corresponds to a transfer allowance for transferring the toner image on the intermediate transfer member to the final transfer member. As this difference is increased, the transfer allowance becomes greater.
If a toner image which has passed through a primary transfer section, which transfers the toner image from the image bearing member to the intermediate transfer member, includes portions having different amounts of adhering toner, unevenness of a charged amount occurs in the toner image on the intermediate transfer member. For example, if the toner image on the intermediate transfer member includes a solid portion and a halftone portion, a charged amount of the solid portion may be lower than that of the halftone portion. Furthermore, for example, in a color image forming apparatus is used, a charged amount of a portion where toner of a plurality of colors is superimposed in the toner image on the intermediate transfer member is lower than that of a portion where toner of only a single color adheres to the intermediate transfer member.
In addition, unevenness of the charged amount in the toner image is sometimes caused by a peeling discharge which occurs downstream adjacent to the primary transfer section in a surface moving direction of the intermediate transfer member after the toner image has passed through the primary transfer section for transferring the toner image from the image bearing member to the intermediate transfer member.
If there is unevenness of a charged amount in the toner image on the intermediate transfer member as described above, the toner image contains portions having different transfer characteristics. If an attempt is made to transfer all the portions having the different transfer characteristics to the transfer member under the same transfer conditions, a range of the transfer condition over which the entire toner image can be transferred at a transfer rate greater than a predetermined value is narrowed. As a result, the transfer allowance for the entire toner image is decreased, and thereby the toner image cannot be transferred stably to the final transfer member.
Referring to FIG. 9, a graph of transfer characteristics is shown with curved lines a and b indicating relationships between a transfer current and a transfer rate for a solid portion and a halftone portion, respectively. Widths indicated by reference numerals A and B in FIG. 9 correspond to ranges of a transfer current which allows respective portions to be transferred to a final transfer member at a transfer rate of 80% or greater, and the transfer allowances are relatively high. However, a range of a transfer current which allows both of these two portions to be transferred at a transfer rate of 80% or greater is narrow as indicated by reference numeral C in FIG. 9 due to a deviation of the two curves a, b, and therefore a transfer allowance is lowered. That is, an overlapping of curves A and B, which is a range which allows both solid and halftone image portions to be effectively transferred, is narrow, which thereby lowers a transfer allowance.
As a further drawback, a background image forming apparatus requires both an intermediate transfer member discharging device, such as a corona discharger, for removing charges on a surface of the intermediate transfer member and a cleaning device, such as a cleaning blade, for removing deposits on the surface of the intermediate transfer member individually, and therefore it is difficult to reduce the cost of such a background device.